1. Field of the Invention
The present invention relates to a method of producing a composite sheet including a ceramic layer and to a method of producing a laminate by using the composite sheet. More particularly, the invention relates to a method of producing a composite sheet that is preferably used for the production of a variety of wiring boards and a wiring board applied to packages for containing semiconductor devices, and is particularly used for the production of ceramic boards having wiring conductors capable of radiating the heat and permitting the flow of heavy currents of power module boards. The invention further relates to a method of producing a laminate such as a wiring board using the composite sheet.
2. Description of the Related Art
In recent years, the semiconductor devices are generating the heat in ever increased amounts accompanying the trend toward highly densely integrating the semiconductor devices.
In order to prevent the semiconductor devices from erroneously operating, it is necessary to use a wiring board which is capable of radiating the heat out of the device.
As for the electrical characteristics, further, a delay in the signals causes a problem as the operation speed increases, and it has been desired to use a wiring conductor having a small wiring conductor loss, i.e., to use a wiring conductor having a low resistance.
As for the wiring board mounting the semiconductor devices, there has been much used a ceramic wiring board using alumina ceramics as an insulating board and depositing a wiring layer of a high-melting metal such as tungsten or molybdenum on the surfaces thereof or in the interior thereof from the standpoint of reliability.
With the wiring layer of a high-melting metal that has heretofore been much used, however, the resistance can be decreased down to about 13 μΩ·cm at the greatest.
In order to flow a large current by lowering the resistance of the wiring conductor in the multi-layer wiring board, therefore, it has been attempted to form a thick film of copper (Cu) or a wiring conductor by non-electrolytic plating on the insulating board that constitutes the multi-layer wiring board.
In the wiring conductor in which the wirings are densely arranged, however, the wire width of the wiring pattern is limited by the area of the multi-layer wiring board and cannot be increased beyond a predetermined width. Besides, with the above method of forming the wiring conductor, it is difficult to obtain a wiring conductor having a sufficient thickness at a low cost within short periods of time without adversely affecting the subsequent steps. Therefore, the above requirement of decreasing the resistance is not satisfied.
In order to flow a large current by lowering the resistance of the wiring conductor, therefore, there have been proposed low-resistance wiring conductors in which through holes are formed in a ceramic green sheet that constitutes the multi-layer wiring board, and the through holes are thickly filled with a wiring conductor paste comprising a low-melting metal having a low electric resistance, such as copper (Cu) or silver (Ag)(see for, for example, Japanese Unexamined Patent Publications (Kokai) Nos. 5-21635 and 63-194).
When the above low-resistance wiring conductor is to be formed, through holes are formed in a predetermined ceramic green sheet, and recessed portions formed by laminating another green sheet are filled with metal paste for wiring by a screen-printing method or a dispenser method.
After the paste has been filled, the drying is effected. At this moment, the surfaces of the portions filled with the paste become rugged such as dented. When a green sheet is laminated, therefore, the laminate becomes defective or is deformed.
In forming complex patterns, further, the paste is poorly filled to cause defective connection to other circuits.
Further, a wiring board of a structure having a gap therein for accommodating an electronic part such as LSI as represented by a substrate for packaging the semiconductor device, has so far been produced by a method that is described below.
Namely, a suitable organic binder is added to a starting ceramic powder prepared at a predetermined ratio, which is, then, dispersed in an organic solvent to prepare a slurry thereof. Then, a ceramic green sheet of a predetermined thickness is formed by a casting method such as doctor blade method or lip coater method. A metal paste obtained by adding an organic binder, a solvent and a plasticizer to a suitable metal powder, is, then, printed and applied on the green sheet in a predetermined wiring pattern by the screen-printing method. Then, through holes are formed by using a micro-drill or a laser beam. The through holes are then filled with a metal paste to form via conductors.
Then, in order to form a cavity for accommodating an electronic part, a through hole is pierced by punching at a predetermined portion of the green sheet.
Then, as shown in FIG. 9(a) which is a view illustrating a conventional step, a plurality of green sheets, i.e., green sheets 21a and 21b in which the through holes 20 are formed are laminated together with other green sheets 21c, 21d and 21e using a suitable adhering solution, and the thus obtained ceramic laminate molded article is fired under predetermined conditions to obtain a board 23 having a cavity 22 for accommodating an electronic part as shown in FIG. 9(b).
In this ceramic board 23, the wiring patterns and via holes must be finely formed from the standpoint of fabricating the substrate in a small size, and it is indispensable to improve precision at the time of lamination.
In the substrate having the cavity 22, however, the pressure fluctuates between the cavity 22 and other portions at the time of laminating the green sheets 21a, 21b having the through hole 20 for constituting the cavity 22 on other green sheets 21c, 21d, 21e, and deformation takes place at the time of lamination. If the pressure is decreased to prevent the deformation, then, the adhesion becomes defective among the green sheets.
The deformation referred to here includes, as shown in FIG. 9, deformation in the horizontal direction that occurs in the green sheets 21a and 21b near the cavity 22 due to pressure in the vertical direction exerted on the laminate of the green sheets 21a to 21e, and deformation resulting from the swollen bottom of the cavity 22 in the green sheet 21c. 
Of them, the swollen bottom of the cavity 22 causes defective bonding when the electronic part such as LSI chip is to be mounted. The swelling of the bottom of the cavity 22 can be decreased by lowering the pressure exerted on the laminate arousing, however, such a problem that the layers are peeled (delamination occurs) as designated at 24 in the periphery of the cavity 22. Further, accompanying the widespread use of portable terminals, in recent years, it has been urged to transmit signals at increasing speeds and at higher frequencies, and to transmit the data in large amounts, while decreasing the sizes, decreasing the background noise level, and decreasing the weight.
As for the parts that are to be mounted, therefore, it has been desired to provide a low-loss material that can cope with high-frequency signals and a wiring substrate using a low-loss conductor.
As a requirement for decreasing the size, further, the parts to be mounted have been fabricated in a modular form, and it has been desired to provide a wiring board fulfilling higher functions.
Among such requirements for fulfilling high functions, it is a modern trend to use such a wiring board as LTCC board made of a material of a low dielectric constant and using a low-loss conductor such as copper or silver, or a board using an organic material, making it possible to incorporate a capacitor by using a paste for enhancing the function and being compounded with a layer of a high dielectric constant and with a layer of a low dielectric constant.
When, for example, the dielectric constant is lowered in a portion of the multi-layer wiring board or a capacitor is formed in order to lower the transmission loss of a portion that corresponds to high-frequency signals, it has been proposed to decrease or increase the dielectric constant of the sheet of a corresponding portion among the plurality of sheets that are laminated (see, for example, Japanese Unexamined Patent Publications (Kokai) Nos. 2002-185147 and 2002-290053).
There has further been proposed to form a portion of a low dielectric constant in a sheet by forming a recessed portion at a predetermined portion of the sheet and filling the recessed portion with a ceramic past of a low dielectric constant or a high dielectric constant (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-97854).
According to the method of laminating different ceramic green sheets, however, the dielectric constant is decreased or increased even up to those portions which are not desired, and limitation is imposed on designing the circuit.
According to the method of filling the recessed portions with the ceramic paste, further, the ceramic paste contains a solvent or the like. After the paste is dried, therefore, a step tends to occur between the surface of the ceramic green sheet and the surface of the paste that is filled. When a multi-layer structure is realized, therefore, defects tend to occur in the laminate.
Further, the portions filled with the ceramic paste and the ceramic green sheet have different molding densities, and it is difficult to equalize the contraction factors at the time of firing. As a result, gaps often develop in the junction portions.